We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

By the answers given, it's clear that you asked the exact opposite question of the one you should have asked, to wit: "When should I use WD-40?" So far, we've learned that it's not a lubricant, it will attack plastic, lacquer (insulation on magnet wire in a motor), etc. It's really just for cleaning water off of things, and happens to work as a weak penetrating lubricant. It's kind of like a Swiss Army Knife in that you could use it for all sorts of things, but you really shouldn't. Use the proper lubricant for the job and you'll be happier in the long run.
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Andrew BealsMar 12 '12 at 19:09

No it's not, really. See other answers.
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tomfanningMar 18 '12 at 21:51

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@tomfanning "WD-40 cleans/degreases, penetrates to loosen up stuck parts, prevents corrosion and is a light lubricant." wd40.com/faqs There are several more mentions of WD-40 being a lubricant on the site. It contains mineral oil, which is a light lubricant.
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xpdaMar 19 '12 at 4:08

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I've seen locksmiths use WD-40 in locks. (I can't vouch for their being good locksmiths though. The one that changed my ignition didn't rekey it to match my keys.)
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aijSep 13 '14 at 20:57

WD-40 isn't actually a true lubricant. WD stands for water displacing. The main use for WD-40 is as a solvent and a rust dissolver. The lubricant-like properties of WD-40 are from rapidly evaporating components. It won't last. True lubricants include things like silicone, grease, Teflon, graphite, etc. WD-40 can be good as a first go at things, it can help clean up rust or other grime and then you can apply your lubricant.

@xpda When the person on the other side of the issue has given more information than you, your comment is very unhelpful. Have any references for that?
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NickCMar 18 '12 at 17:10

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@NickC "WD-40 cleans/degreases, penetrates to loosen up stuck parts, prevents corrosion and is a light lubricant." wd40.com/faqs There are several more mentions of WD-40 being a lubricant on the site. It contains mineral oil, which is a light lubricant.
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xpdaMar 19 '12 at 4:05

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Just because it contains a small amount of lubricant does not make WD-40 a lubricant itself. It contains many other solvents which counteract the lubricating properties of the mineral oil such as acetone.
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Cody CMar 30 '12 at 15:24

WD-40 does a lot of things, unfortunately this means that it does not do any one thing well.

As a lubricant WD-40 is very thin and runny. WD-40 will lubricate for a short time, but will quickly run off. This is a side effect from it being a penetrant. If it was thicker for better lubrication it would not penetrate well.

WD-40 is a GREAT quick-fix and one-can option. I use it in that role, it gets things going long enough to let me finish the current task and get back to the shop. Once there I can get to my real penetrants (Kroil, PB Blaster) and my real lubricants (oil, white lithium) and repair the problem.

Bonus tip: wrap your WD-40 can with several turns of duct-tape. You now have two of the best quick-fix wonders in one package.

This is also the reason you do not ever use it in locks, especially pin tumblers. My brother (a locksmith) used to swear at people who tried to "clean" a tumbler with WD-40. And he's not a sweary person.
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staticsanMar 9 '12 at 2:11

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@PaulD.Waite A very light oil is good for hinges. a common brand is called 3-in-1 Oil. While not as easy to apply, vegetable oil (the kind used for cooking) will work and you can even get it in a spray can (brand name Pam) but don't get the kind with flour or flavoring.
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Christopher EstepMar 9 '12 at 22:35

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3-in-1 oil will attract dirt and dust just as bad as WD-40. And for matter, so will any other type of oil or grease. For locks you can use graphite powder, for door hinges the natural polishing action of using them is enough.
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Jonathan AllenMar 17 '12 at 7:10

Rubik's cubes. Serious cubers will lube their cubes with Jigaloo, CRC silicone, or even vaseline. But, will point and laugh at noobs who use WD-40, as it melts the cube, likely seizing it. Yet people keep doing it.

Try grease or heavy gear oil, that will really attract dirt. I like WD-40 or 3-in-1 oil for my bike specially because it doesn't last.
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Jonathan AllenMar 17 '12 at 7:12

WD-40 will not ruin a bike chain. It will loosen up a rusty chain. Rust and dirt will ruin a bike chain. However, wax or teflon lubricants will keep a bike chain cleaner (and you can go faster).
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xpdaMar 17 '12 at 15:18

Lock cylinders. Never in lock cylinders. Doorknobs, padlocks, etc. - the residue left will just attract dirt & grit, and prematurely wear down the works.

This was given me by a college friend whose dad worked on the maintenance staff - so they did a lot of locks!! It probably took years to see the pattern, but any lock they had shot with WD-40 eventually failed before un-WD'ed locks.

I read an article once by a clock repairman who described how bad WD-40 is for precision clockworks. The main reason has been given already: it attracts dirt, which acts as an abrasive and eventually gums up the action.

For the same reason you wouldn't use it on door hinges, it should never be used for chair and sofa mechanisms. Because WD-40 attracts dust and turns things black, over time that dirt accumulation within the moving parts will build up and eventually start to fall off onto carpeting creating a terrible black mess that is nearly impossible to remove.